This disclosure generally relates to non-destructive inspection equipment and methods, and relates more particularly to methods and apparatus for inspecting structures made of composite material.
Non-destructive inspection of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or flaws in the structure. Non-destructive inspection is also used in the initial fabrication of the aircraft's structural components. It is used to assure that a part was fabricated correctly and that foreign material is not embedded within the part. Inspection may be performed during manufacturing of a structure and/or after a structure has been put in service
Non-destructive inspection (NDI) may be performed on stiffened composite parts of an aircraft. The stiffener of the stiffened part may be made of a composite material such as carbon fiber-reinforced plastic (CFRP). A composite stringer attached to a composite fuselage is but one example of such a stiffener.
The quality of a stiffened part can be determined non-destructively by ultrasonic testing. A stiffened part can be inspected ultrasonically by a probe including one or more shoes that hold respective ultrasonic transducer arrays. During NDI, the shoes are pressed against respective external surfaces of the stiffened part, the transducers are acoustically coupled to the stiffened part (e.g., using water), and the probe is moved incrementally along the length of the stiffened part. As the probe is being moved, the transducer arrays operate in pulse/echo mode to generate pulsed ultrasonic waves, which propagate into the stiffened part. Reflected ultrasonic waves are returned to and detected by the transducer arrays to provide data indicative of the presence of cracks, voids, delaminations, etc. in the stiffened part. Data acquired by the transducer arrays is typically processed by a computer system, and the processed data may be presented to a user via a computer monitor. A data acquisition device and data handling software may be used for collection and display of inspection data, such as displaying the data on a computer monitor as an image representation of the structure under inspection, such as a hat stringer, supplemented with corresponding color and/or graphical data of the inspection to permit examination by a qualified inspector.
A typical NDI probe has sensing elements, such as ultrasonic transducers, which are placed in proximity to the surface to be inspected. In many cases, the inspected part has multiple surfaces of different shapes and orientations, requiring the use of multiple transducer arrays. This enables the inspection of the structure to proceed more rapidly and efficiently, thereby reducing the costs associated with the inspection. Typically, different structures are inspected using respective transducer arrays which have been specifically designed to provide transducer alignment (position and orientation with respect to the surfaces of the structure) and scan coverage for the entire structure.
The aerospace industry has been moving from manual manufacturing by skilled workers to the use of automated machinery. This is particularly the case in the field of non-destructive inspection of composite structures. Automated inspection systems have been developed as an alternative to manual and semi-automated inspection techniques. Such systems typically employ a manipulator (e.g., overhead gantry, multi-axis scanner, or robot) that scans the NDI end effector along the part being inspected. For single-sided inspection methods, such as pulse echo ultrasonic inspection, a single-arm robotic device having six degrees of freedom may be used to position and move an NDI end effector, such as a pulse echo ultrasonic inspection device, attached to the end of the robot arm. The part to be inspected may be mounted to a holder which is rotatable about an axis. Thus a total of eight degrees of freedom allow for complete inspection of the part. The eight degrees of freedom are controlled by a robot controller in accordance with trajectories generated from a digital model of the inspected part.
Various systems have been employed for inspecting fuselage and wingbox stiffeners (also known as “hat stringers”) having a trapezoidal profile with two corner radii. Some systems have three transducers which are respectively employed to inspect the corners and a central cap portion connecting the corners. Each transducer has its own ultrasonic setup technique and its own NDI qualification that it has to meet. Data from three transducer arrays has to be stitched together to provide a continuous C scan data display. Such three-transducer systems for inspecting a stringer cap may have a large, expensive and complex configuration and are not optimal for inspecting rounded cap stringers.
The foregoing systems may be further equipped with four transducer arrays for NDI of the lower outside radii (LOR) and stringer sides (SS). In such seven-transducer systems, the robot will keep the three transducer arrays that scan the cap and corners aligned, but the other transducer arrays for the LOR and SS are subject to many dimensional factors that hinder their positioning. They must be able to adapt to the following variables: stringer height, stringer thickness, fuselage ply drops, asymmetrical stringer cross section, and irregular surface conditions due to process problems (porosity, resin bubbles, etc.). If the LOR and SS transducer arrays are unable to adequately adjust to the foregoing variables, it may become necessary to perform a rescan.
It would be advantageous to provide a self-aligning automated system for inspecting a rounded cap stiffener in a single continuous NDI procedure while reducing or eliminating rescan of the stiffeners.